The varied layers of soil that you see when driving through a road cutting are known as a soil profile. The dark layer near the surface is the topsoil. It is usually dark because it contains organic matter – as well as most of the nutrients for plant growth. This is the layer with most plant roots. Below this is the subsoil, which can vary a lot in depth. It contains few nutrients, but can be a valuable source of moisture for plants during dry periods.
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Natural Sciences Image Library of New Zealand
Photograph by G. R. Roberts
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